Natural gas and other methane-rich feedstocks are frequently produced in regions distant from the location where such fuels will be finally utilized. The problem of transportation of natural gas from remote production sites to other sites of utilization is particularly acute when the natural gas must be shipped overseas. In such instances, absent a pipeline, the costs of transportation require that the natural gas be liquefied. The liquefaction of natural gas is energy intensive and systems for performing this liquefaction must be extremely efficient in order to maintain the competitive economics of natural gas as fuel being transported over significant distances. Various processes for the liquefaction of natural gas or the separation of natural gas liquids, i.e. hydrocarbons heavier than methane, have been set forth in the prior art.
In U.S. Pat. No. 3,292,380, a process for removing condensibles from a hydrocarbon gas stream is set forth in which the feedstock is heat exchanged against the overhead from a distillation column before being separated into gas and liquid phases, the gas phase of which is expanded in a turbine and delivered to the distillation column. A portion of the liquid phase is also supplied to the column. An overhead gas phase which is not liquefied is drawn off from the distillation column and a heavier hydrocarbon such as ethane and LPG is drawn off as a bottom stream from the column. This patent is directed only to the removal of condensibles and not to the liquefaction of natural gas.
U.S. Pat. No. 4,004,430 also discloses a process for removing natural gas liquids from a methane-rich stream. The methane-rich gaseous product is separated from the natural gas liquids product in a cryogenic distillation column. Again, the methane-rich product is not liquefied.
In U.S. Pat. No. 4,061,481, a process is disclosed for the separation of condensible hydrocarbon liquids from gaseous hydrocarbon components in a distillation column. The feedstock is heat exchanged against the overhead of the distillation column prior to and after being expanded to a lower pressure. Liquefaction of the overhead methane-rich stream from the distillation column is not set forth.
U.S. Pat. No. 4,065,278, having the same assignee as the present invention, is directed to a natural gas liquefaction process wherein condensible higher hydrocarbons are removed from the natural gas stream prior to liquefaction of the methane-rich gas. In this patent, an additional heat exchange bundle is utilized to provide the initial cooling of the methane-rich overhead from a distillation column wherein the additional heat exchange bundle utilizes a low temperature refrigeration.
U.S. Pat. No. 4,203,741 discloses a separator system for hydrocarbon gas feed streams. The feed stream is split into a plurality of feeds to a separation or distillation column. One of the feed streams is expanded and heat exchanged against the overhead from the column. The process produces natural gas liquids and a vapor product which may be methane-rich.
In a paper presented at the 58th annual GPA convention on Mar. 19 through 21, 1979 at Denver, Colo. titled OPTIONS FOR ETHANE REJECTION IN THE CRYOGENIC EXPANDER PLANT, by Jerry G. Gulsby, an ethane rejection plant is set forth in which a hydrocarbon inlet gas is heat exchanged against a demethanizer column overhead stream and expanded before being introduced into the demethanizer. The overhead stream from the demethanizer is recompressed but is not liquified.
In another article appearing in the Oil and Gas Journal of Mar. 13, 1972 titled CRYOGENIC PROCESSING HAS WORKED FOR MOBIL by R. D. Parker, a cryogenic system for separating hydrocarbons heavier than methane from methane is set forth in which the heavier hydrocarbons are liquified. The methane fraction of the feed gas being treated is not liquefied. At least a portion of the feed to the demethanizer column is exchanged against an overhead stream from such column.
The prior art fails to disclose the advantage of the present invention wherein a natural gas liquefaction process is provided with expanded feed being added to the top of the scrub column at relatively high pressure and the methane-rich overhead is liquefied in a two bundle heat exchanger in an efficient manner, wherein liquid feed or reflux to the column is provided by refrigeration power from a high level refrigerant and the isentropic expansion of the feed and not by low level refrigeration.
The prior art also fails to disclose another advantage of the present invention wherein in a combined separation and liquefaction process for natural gas streams in which heavier hydrocarbons are separated from natural gas before the methane-rich natural gas is liquefied, the methane-rich overhead from the separatory or scrub column is heat exchanged in an intercooler against the feed stream being introduced into the column. This provides for increased efficiency of operation of a system wherein natural gas liquid recovery and methane liquefaction are combined.